Continuous anodic oxidation method for aluminum and alloys thereof

ABSTRACT

A METHOD OF CONTINUOUSLY FORMNG AN OXIDE FILM ON ALUMINUM AND ALLOYS THERREOF BY ANODIC OXIDATION WITHOUT RESORTING TO A CONVENTIONAL PROCESS USING AN ELECTROLYTIC CELL, AND AN APPARATUS FOR PRACTICING THE METHOD WHICH IS COMPACT IN SIZE AND IS CAPABLE OF NOT ONLY HIGH CURRENT DENSITY TREATMENT AND HIGH SPEED TREATMENT BUT ALSO VARIOUS TYPES OF ELECTROLYSIS.

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CONTINUOUS ANODIC OXIDATION METHOD FOR ALUMINUM AND ALLOYS THEREOF 6Sheets-Sheet 2 Filed Sept. 14, 1970 FIG.

p 1972 TAKESHI HAMABE EI'AL 3,692,640

CONTINUOUS ANODIC OXIDATION METHOD FOR ALUMINUM AND ALLOYS THEREOF FiledSept. 14, 1970 6 Sheets-Sheet I5 Sept. 19, 1972 TAKESHl HAMABE ETA.3,692,640

CONTINUOUS ANODIC OXIDATION METHOD FOR ALUMINUM AND ALLOYS THEREOF FiledSept. 14, 1970 6 Sheets-Sheet 4.

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p 19, 1.972 TAKESHI HAMABE ET 3,692,640

CONTINUOUS ANODIC OXIDATION METHOD FOR ALUMINUM AND ALLOYS THEREOF FiledSept. 14, 1970 I .6 Sheets-Sheet 5 Sept. 19, 1972 TAKESH| HAMABE ETAL3,692,640

CONTINUOUS ANODIC OXIDATION METHOD FOR ALUMINUM AND ALLOYS THEREOF FiledSept. 14, 1970 6 Sheets-Sheet 6 V 20 p, I

ufiiteasmes Patent 01 3,692,640 Patented Sept. 19, 1972 ice US. Cl.20428 7 Claims ABSTRACT OF THE DISCLOSURE A method of continuouslyforming an oxide film on aluminum and alloys thereof by anodic oxidationwithout resorting to a conventional process using an electrolytic cell,and an apparatus for practicing the method which is compact in size andis capable of not only high current density treatment and high speedtreatment but also various types of electrolysis.

The present invention relates to a method and apparatus for continuouslyoxidizing by anodic oxidation the surface of elongate articlesconsisting of aluminum or an aluminum alloy (hereinafter referred to asaluminum as a whole), i.e. aluminum foils, aluminum strips, aluminumwires, etc.

Anodic oxidation of aluminum has heretofore been conducted at a lowcurrent density of several amperes per 1 dm.'. At such a degree ofcurrent density, the occurrence of the so-called yellowing of film andnonuniform current distribution can be prevented simply by propertemperature control and suflicient stirring of electrolyte. However, therecent tendency in the art of anodic oxidation is towards high currentdensity anodic oxidation with a view to high speed anodic oxidationtreatment and the problems of film yellowing and nonuniform currentdistribution are being highlighted. These problems are difficultsubjects which cannot be solved unless the electrode construction andanodic oxidation process are drastically changed. The so-calledyellowing of film is caused by the heat which is generated in theprocess of anodic oxidation, and is closely related to currentdistribution.

The heat generated in the anodic oxidation is attributed to the heatgenerated by the reaction between aluminum and oxygen, and the Jouleheat generated during the passage of a current through the oxide film,electrolyte, cathode and aluminum. In the ordinary anodic oxidationconducted in an electrolytic cell, the heat generated is substantiallyentirely transferred to the electrolyte in the electrolytic cell and,therefore, the temperature of the electrolytic system can be maintainedconstant, by cooling the electrolyte. However, with the current densityat which the anodic oxidation is conducted becoming higher and higher,such a method is not capable of sufficiently cooling the electrolysissystem and non-uniform temperature distribution will result. At acurrent density higher than a certain value, yellowing of the filmoccurs, making it impossible to obtain a normal oxide film. This isbecause the intensity and uniformity of stirring of the electrolyteundergo a certain limitation.

The present invention consists in an anodic oxidation method which isproposed with a view to obviating the above-described problems, and inwhich the socalled electrolytic cell with a predetermined quantity ofelectrolyte stored therein, as used heretofore, is not used and hencethe electrolytic portion can be sufiiciently cooled, providing for ahigh current density anodic oxidation at high efficiency.

The anodic oxidation method according to the present invention isadapted for continuous oxidation of the entire area of at least one sidesurface of an elongate article of aluminum, basically by passing thearticle continuously in opposed relation to a cathode device. Thecathode device is composed of a hollow container which is provided withinlet ports for continuously feeding an electrolyte therein and has acathode therein. One side of the hollow container is open and coveredwith a liquidpermeable material. The elongate article of aluminum ispassed in front of the liquid-permeable material, with a predeterminedinterval therebetween, or is passed in contact with the same. The spacebetween the liquidpermeable material and the elongate article ofaluminum is filled with the electrolyte which has been fed into thehollow container through the inlet ports and uniformalized in liquidpressure by the liquid-permeable material during passage therethrough.In this case, the space between the cathode and the liquid-permeablematerial interior of the cathode device is of course filled with theelectrolyte. Therefore, the cathode and the elongate article of aluminumare electrically connected by the electrolyte through theliquid-permeable material. A voltage is impressed on the cathode and theelongate article of aluminum under such condition, whereby the surfaceof the aluminum is oxidized by anodic oxidation.

The present invention will be described in further detail hereunder withreference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view illustrating an embodiment of thepresent invention;

FIG. 2 is a cross-sectional view illustrating another embodiment of theinvention;

FIG. 3 is a cross-sectional view illustrating still another embodimentof the invention, in which a plurality of the electrolytic units shownin FIG. 1 and cooling sections are arranged alternately;

FIG. 4 is a perspective view, partially in section, of another form ofthe cathode device shown in 'FIG. 1; and

FIGS. 5 and 6 are perspective views of still other forms of the cathodedevice, which are adapted for the anodic oxidation of aluminum wires,respectively.

Referring first to FIG. 1, there is shown in cross-section anelectrolytic unit for practicing the method of this invention. The termelectrolytic unit as used herein refers to a unit by which anodicoxidation is effected and which includes aluminum to be oxidized, acathode device and an electrolyte. In the arrangement of FIG. 1reference numeral 1 designates inlet ports through which the electrolyteis continuously fed into the cathode device by a pump 10 or from a tank(not shown) disposed above said cathode device. Reference numeral 2designates a cathodeconstituting electrode plate and a lead 3 thereof isconnected to one terminal of an anodic oxidation power source (notshown). Reference numeral 4 designates the electrolyte filling theinterior of the cathode device, and 5 designates a liquid-permeablematerial constituting one side of the cathode device. Theliquid-permeable membrane 5 serves as a discharge port 6 for theelectrolyte. Reference numeral 7 indicates the electrolyte dischargedfrom the cathode device through the liquid-permeable material 5, whichconstantly fills the space formed between an aluminum 8 to be treatedand the cathode device. As described above, the cathode device iscomposed of the inlet ports 1 for the electrolyte, the cathode 2 and theliquid-permeable material constituting the discharge port for theelectrolyte.

The electrolyte first flows into the cathode device through the inletports 1 as indicated by the arrows and then towards the discharge port 6to be discharged from the cathode device through the liquid-permeablematerial 5. The aluminum 8 to be treated is continuously passed in frontof the liquid-permeable material 5 in the direction of the arrow or inan opposite direction, with a predetermined interval therebetween.Therefore, the electrolyte 7 discharged through the liquid-permeablematerial 5 flows down along the surface of the aluminum. As a result, acurrent flows between the cathode and the aluminum through theelectrolyte and the surface of the aluminum is oxidized by anodicoxidation. The electrolyte which has been used for the electrolysisreaction while filling the space between the aluminum and the cathodedevice, immediately flows down into a tank 9 disposed below theliquid-permeable material 5, cooled in said tank and again fed into thecathode device through the pump 10.

The apparatus of FIG. 1 is adapted for anodic oxidation of only one sidesurface of the aluminum. Where both side surfaces of the aluminum aredesired to be oxidized by anodic oxidation, this can be attained simplyby providing another apparatus of the same construction on the oppositeside of the aluminum.

As will be clearly understood from the foregoing description, since theelectrolyte discharged from the cathode device is used for electrolysiswhile flowing downwardly and the aluminum is moving per se, in no caseis the electrolyte allowed to remain stationary at any point on thealuminum. Therefore, the heat generated on the filmforming surface ofthe aluminum can quickly removed therefrom, and hence yellowing of thefilm hardly occurs in the method of this invention.

Although many materials are used for the liquid-permeable material whichcovers the discharge port for electrolyte, an acid-resistant metalscreen, an ethylene tetrafluride fabric or a polypropylene fabric issuitably used. The mesh of the liquid-permeable material is selectedcase by case within the range of 0.01-1 mm., in relation with thedesired rate of discharge of the electrolyte.

By using the cathode device described above, various anodic oxidationmethods can be considered, besides the one described above. The firstone of these methods is to pour a cooling liquid onto the surface of thealuminum opposite to the surface on which an oxide film is being formed,so as to enhance the cooling effect of the surface being subjected toanodic oxidation. This arrangement is shown in FIG. 2 in cross-section.In FIG. 2, reference numeral 11 designates the cathode devices by whichboth surfaces are oxidized by anodic oxidation respectively and each ofwhich forms an electrolytic unit together with the aluminum 8 and theelectrolyte 7. Reference numeral 12 designates a cooling liquid pouredonto the surface of the aluminum opposite to the surface being subjectedto anodic oxidation, to cool the electrolytic unit. In this method also,one or a plurality of the cathode devices are provided on one side ofthe aluminum where only one side of the aluminum is desired to beoxidized, or on each side of the aluminum where both sides of thealuminum are desired to be oxidized. The cooling liquid is poured ateach electrolytic unit.

The second method is to provide the electrolytic units and the coolingunits alternately for the purposes of enhancing the cooling effect ofthe electrolytic units and forming cracks in the oxide films formed byanodic oxidation. This arrangement is shown in FIG. 3 in cross-section.In FIG. 3, reference numeral 11 designates the cathode devices, 14 thecooling units for ejecting a cooling liquid against the aluminum 8 tocool the latter, and 4 the electrolyte in the respective cathodedevices. The cooling liquid serves, not only to cool the aluminum butalso to form cracks in the oxide films formed. Namely, by rapidlycooling the surfaces of the aluminum which is heated to an of cracksformed, and oxide films excelling in flexibility can be obtained.

The third method is to create an electrolyte temperature differencebetween adjacent ones of two or more electrolytic units provided alongthe direction of travel of the aluminum, for the purpose of formingcracks in the oxide films formed on the surfaces of the aluminum. In theprocess of electrolysis, a large amount of heat is gen erated on thesurfaces of the aluminum being subjected to electrolysis, and hence thealuminum substrate and the oxide films formed thereon are elevated to aconsiderably high temperature. If, in this case, a temperaturedifference is created in the electrolyte in the respective electrolyticunits, cracks are formed in the oxide films due to thermal stress. Thesecracks are useful for improving the flexibility of the oxide films.

On the other hand, the construction and shape of the cathode device arewidely variable. FIG. 4 is a perspective view, partially shown insection, of a cathode device which is particularly advantageous inproviding a uniform current distribution in the treatment of aband-shaped aluminum. In FIG. 4, reference numeral 22 designates acathode, 1 inlet ports for feeding the electrolyte into the cathodedevice therethrough, 5 the liquid-permeable material covering thedischarge port of the cathode device, 8 a band-shaped aluminum to betreated, 21 an insulating material constituting a part of the cathodedevice, and 25 a liquid-permeable material for further enhancing theuniformity of current distribution in both vertical and horizontaldirections. The width of the cathode is equal to or smaller than thewidth of the aluminum to be treated and the vertical length thereof isshorter than the vertical length of the electrolyte discharge port. Thefront surface of the cathode is corrugated, so as to increase theeffective area thereof. On the inside or outside of the liquid-permeablematerial at the electrolyte discharge port of the cathode device areprovided the other liquid-permeable material for contact with thelongitudinally spaced edge portions and the opposite edge portons of thealuminum, so as to avoid concentration of current to the longitudinaland transverse edges of the aluminum due to the edge effect of electriccurrent.

FIG. 5 and 6 are perspective views of cathode devices respectively whichare most adapted for anodic oxidation of aluminum wires. Referring firstto FIG. 5, the cathode device shown in a double-walled cylindrical bodyhaving an inner wall consisting of a liquid-permeable material 5 and anouter wall consisting of a metal plate 2 which constitutes a cathode.The inner and outer walls are connected at the upper and lower edgesthereof by annular metal plates which serve as cathode, or othermaterial as at 20, so as to define an annular space between the innerand outer walls, and electrolyte inlet ports 1 are provided on the upperannular plate 20. An electrolyte introduced into the cathode devicethrough the inlet ports 1 is ejected towards an aluminum wire 8 to betreated, through the liquid-permeable material 5. Thus, the aluminumwire 8 is oxidized by anodic oxidation, while continuously moving in thedirection of the arrow. The electrolyte 7 is continuously ejectedthrough the liquid-permeable material 5 and drops down under gravity, sothat the heat is removed from the electrolytic unit highly efficiently.The ring shape of the cathode is advantageous is providing a uniformelectric field for the aluminum wire, and makes a great contribution tothe improvement in uniformity of the oxide film formed. Turning now toFIG. 6, there is shown a cathode device of the same type as that of FIG.5 but the inner wall thereof consists of the metal plate or othermaterial and the outer wall thereof consists of the liquid-permeablematerial. Aluminum wires 8 are oxidized by anodic oxidation during theirpassage along the surface of the liquid-permeable material.

Now, the present invention will be further illustrated by way ofexamples thereof.

EXAMPLE 1 charge port. The interval between the aluminum strip and thecathode was 5 mm. A 30 weight percent sulfuric acid solution was used aselectrolyte and the temperature thereof was maintained at 30 C. by acooling device. The electrolyte was supplied into the cathode device atthe rate of 40 l./min. The anodic oxidation was carried out at a currentdensity of 200 a./dm. by continuously conducting a quantity ofelectricity at the rate of 0.5 ah./dm. As a result, a uniform oxide filmhaving a thickness of 6.3 1. was formed over the entire area of bothsurfaces of the aluminum strip.

EXAMPLE 2 A continuous anodic oxidation was conducted, using the methodillustrated in FIG. 2. The aluminum strip, the cathode and theelectrolyte used were the same as those mentioned in Example 1. Thesulfuric acid solution used as the electrolyte, was concurrently used asthe cooling liquid at the same temperature. The cooling liquid dischargeport was formed in a rectangular shape so that it may be ejecteduniformly against each side of the aluminum strip.

During the anodic oxidation, the direct current was supplied to give aquantity of electricity at the rate of 0.5 ah./dm. and the currentdensity was varied from 200 to 600 a./dm. An excellent oxide film havinga thickness of 6.3 and a dielectric breakdown voltage of 220 v. A.C. wasformed by anodic oxidation in the current density range up to 560 a./dm.

EXAMPLE 3 An anodic oxidation of an aluminum wire having a diameter of0.6 mm. was carried out continuously, using the cathode device shown inFIG. 5. Lead was used for the cathode and a polypropylene fabric wasused for the liquid permeable material at the electrolyte dischargeport. The inner diameter of the cathode device was 15 mm. and the outerdiameter thereof was 80 mm. An electrolyte consisting of 30 weightpercent sulfuric acid solution was supplied to the cathode device bymeans of a pump, and the temperature thereof was maintained at 30 C. Theanodic oxidation was conducted at the supply rate of a quantity ofelectricity of 0.5 ah./dm. and a current density of 600 a./dm. and auniform, excellent oxide film having a thickness of 8 was obtained.

As may be clearly understood from the foregoing description, when acontinuous anodic oxidation of an elongate article of aluminum by themethod of this invention, the heat generated at the area of electrolysisincident to the anodic oxidation reaction can be efficiently removed andanodic oxidation can be attained at a high speed. Therefore, theso-called yellowing of the film can be completely avoided, even underelectrolysis at a high current density. Thus, a high speed electrolysisbecomes possible and also the size of the anodic oxidation apparatus canbe reduced, since the method of the instant invention does not require alarge electrolytic cell.

It is also to be noted that, by practicing the method of this invention,an oxide film of improved flexibility can be obtained because cracks canbe formed in the film only by the anodic oxidation treatment.

Thus, the present invention is of great industrial advantage.

What is claimed is:

1. A continuous anodic oxidation method for aluminum, comprisingcontinuously feeding a sulfuric acid solution into a device consistingof a hollow container having a cathode, at least one inlet port, adischarge port, and a liquid-permeable material covering said dischargeport, through said inlet port and discharging the same from saiddischarge port through said liquid-permeable material at a rate of about0.67 1./cm. /cm. /min.: passing bandor rod-shaped aluminum along thesurface of said liquid-permeable material in opposed relation thereof;and impressing a voltage across said cathode and said aluminum at acurrent density of about 200600 amps/ m? at about 30 C., whereby thesurface of said aluminum is oxidized by anodic oxidation.

2. A continuous anodic oxidation method for aluminum according to claim1 wherein said device is provided on one side of said aluminum band andcooling liquid is poured onto the surface of the aluminum band oppositeto the surface of same facing said device.

3. A continuous anodic oxidation method for aluminum according to themethod of claim 1 wherein two of said devices are provided, one on eachside of said aluminum band at points removed from each other so thatsaid two devices are not opposed to each other.

4. A continuous anodic oxidation method for aluminum, as defined inclaim 1, wherein a plurality of said devices and a plurality of coolingliquid pouring devices are arranged alternately on at least one side ofthe aluminum along the direction of travel of the aluminum.

5. A continuous anodic oxidation method for aluminum, as defined inclaim 1, wherein at least one device which discharges the electrolyte ata relatively high temperature and at least one cathode device whichdischarges the electrolyte at a relatively low temperature arealternately arranged along the direction of travel of the aluminum.

6. A continuous anodic oxidation method for aluminum according to claim1 wherein said discharge port is in the side of said container and saidbandor rod-shaped aluminum is passed vertically along the surface ofsaid liquid-permeable material in opposed relation thereto.

7. A continuous anodic oxidation method according to claim 1 whereinsaid liquid-permeable material is an acidresistant metal screen, anethylene tetrafluride fabric or a polypropylene fabric having a meshsize of from 0.01 to 1 mm.

References Cited UNITED STATES PATENTS 1,068,410 7/1913 Chubb 20458 X1,117,240 11/1914 Presser 20458 X 2,930,739 3/1960 Burnhour 20458 X2,989,445 6/1961 Lloyd et a1. 20428 FREDERICK C. EDMUNDSON, PrimaryExaminer U.S. Cl. X.R. 20458, 206

